The invention relates to an arrangement for optically measuring the distance between a surface and a reference plane, comprising a radiation-source unit for illuminating an area on the surface, a radiation-sensitive detection system for converting the radiation reflected from the surface into an electric signal, and an optical system, arranged between the surface and the detection system, for concentrating radiation reflected from the surface on the detection system. The invention also relates to an apparatus for determining the positions of components on a support, which apparatur comprises a system for determining the distance between the surface of the support carrying the components and a reference plane. The optical system may comprise a system of lenses and/or mirrors and converts a diverging beam into a converging or parallel beam.
German patent specification No. 23 55 185 describes an apparatus in which, for measuring the thickness of an object, the distance between the surface of this object and a reference plane is determined. In the known apparatus a narrow laser beam is perpendicularly incident on the surface of the object via a mirror to form a radiation spot on the surface. A part of the radiation originating from the surface is received by an optical system and is concentrated to form a radiation spot in the plane of two adjacent radiation-sensitive photodiodes. The position of the radiation spot on the surface and, consequently, the position of the image formed on the photo-diodes is changed by moving the mirror until equal amounts of radiation are incident on both photodiodes. The position of the mirror is now related directly to the thickness of the object.
The apparatus described in said German patent specification No. 23 55 185 comprises a movable mirror whose speed of movement imposes an upper limit on the attaninable measurement frequency. Another disadvantage is that close to steep slopes the height of a surface to be measured cannot always be measured correctly owing to shadow effects.